A carefully controlled, large-scale field experiment will evaluate the extent to which the amount of interviewer training and supervision affects the quality of data they collect in health surveys. At the same time, a detailed model of the way that interviewers' training affects the reporting of health survey respondents will be assessed. Interviewers will be trained and supervised with varying degrees of intensity, at varying cost. They will then be carefully evaluated in terms of their knowledge of their job, their job performance on a variety of dimensions and the reliability of the data they collect. The findings will provide guidelines for researchers and for those purchasing health services research about the appropriate level of training and supervision for various objectives. In addition, the project will provide extensive knowledge about the role of the interviewer, in the context of total survey design, in producing error in health surveys. The result should be a major contribution to cost-effective survey design and to the overall improvement in quality of health-related survey data.